How Does Lactose Get Digested?
When you drink a glass of milk, your body begins a remarkable process that transforms lactose into nutrients your cells can use. Although lactose is often discussed in relation to lactose intolerance, for most infants and many adults it serves as an important source of both energy and building blocks for growth.
Let's follow the journey of lactose through the digestive system.
Step 1. Lactose enters the small intestine
Lactose is the main carbohydrate found in the milk of mammals. It is a disaccharide, meaning it is made from two smaller sugars joined together:
Glucose
Galactose
Because these two sugars are linked together, the body cannot absorb lactose directly. First, it must be broken apart.
Step 2. Lactase breaks lactose apart
Along the surface of the small intestine are millions of tiny finger like projections called villi. On these villi is the brush border, which contains the enzyme lactase (also known as lactase phlorizin hydrolase).
Lactase acts like molecular scissors, splitting lactose into its two component sugars:
Lactose → Glucose + Galactose
As described in Lactose: Evolutionary Role, Health Effects, and Applications:
"Lactose is cleaved by the enzyme lactase phlorizin hydrolase into glucose and galactose in the brush border of the small intestine."
Once separated, these sugars are small enough to be absorbed.
Step 3. Glucose and galactose enter the intestinal cells
The body does not simply allow these sugars to drift into the bloodstream. Instead, it uses specialized transporter proteins.
The transporter called SGLT1 actively moves both glucose and galactose from the intestine into the intestinal cells. This process uses sodium to help carry the sugars across the cell membrane.
Once inside the cell, another transporter called GLUT2 releases the sugars into the bloodstream, where they travel throughout the body.
Step 4. The liver processes the sugars
After entering the bloodstream, glucose and galactose are transported to the liver.
Here, glucose can:
Be used immediately to produce energy.
Be stored as glycogen for future energy needs.
Galactose follows a slightly different route. Through a series of biochemical reactions known as the Leloir pathway, it is converted into glucose, allowing it to be used for energy as well. The liver can also use galactose to produce important biological molecules needed during growth and development.
Step 5. Not all galactose is absorbed
Interestingly, researchers suggest that a small amount of galactose may escape absorption and continue into the large intestine.
There, beneficial gut bacteria such as Bifidobacteria and Lactobacilli can use it as a food source.
These bacteria help maintain a healthy gut microbiome and may contribute to digestive health. This has led scientists to propose that lactose may provide benefits beyond simply supplying calories.
Why is this important?
One fascinating idea discussed in Lactose: Evolutionary Role, Health Effects, and Applications is that lactose may have evolved because it performs two valuable roles at the same time.
Glucose provides immediate energy.
Glucose is rapidly absorbed and fuels growing tissues throughout the body.
Galactose does much more than provide energy.
Galactose can be converted into glucose, but it also contributes to the production of important molecules involved in brain development and may help nourish beneficial gut bacteria when some reaches the colon.
This combination gives lactose a unique advantage over many other carbohydrates found in milk.
The journey of lactose
You drink milk containing lactose.
Lactase in the small intestine splits lactose into glucose and galactose.
SGLT1 transports both sugars into the intestinal cells.
GLUT2 releases them into the bloodstream.
The liver stores or converts the sugars for energy.
A small amount of galactose may reach the colon, where it can support beneficial gut bacteria.
Key Takeaway
Lactose is much more than just "milk sugar." It is a carefully designed carbohydrate that provides immediate energy through glucose while galactose supports growth, metabolism, and potentially the developing gut microbiome. This dual role helps explain why lactose has remained the primary carbohydrate in mammalian milk throughout evolution.
References
Paques, M., & Lindner, C. (Eds.). (2019). Lactose: Evolutionary Role, Health Effects, and Applications. Academic Press. In particular, Chapter 2: Digestion, Absorption, Metabolism, and Physiological Effects of Lactose by Andrew Szilagyi.
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